1. Field of the Invention
The invention relates to a method for preparing amorphous, hydrogenated carbon having a low interfacial state density, and to amorphous, hydrogenated carbon prepared according to this method, as well as to its application.
2. Description of Related Art
Layers of amorphous, hydrogenated carbon, in short a-C:H, can be produced according to a PECVD method ("Plasma Enhanced CVD") from a, for example, radio-frequency excited plasma (the transmitter being capacitively coupled). Generally, these types of layers have bulk-defect concentrations at the Fermi [characteristic energy] level of 10.sup.16 to 10.sup.20 cm.sup.-3 (c.f.: "Thin Solid Films", vol. 182 (1989), pp. 63-78; "Proc. Electrochem. Soc. 91-8-Proc. Int. Symp. Diamond Mater., 2nd", 1991, pp. 645-652; R. E. Clausing et al. "Diamond and Diamond-like Films and Coatings", NATO ASI Series B (Physics), vol. 266, Plenum Press, New York 1991, pp. 427-437). Such defects can--as as in the case of amorphous silicon (a-Si)--be traced back to unpaired electrons, i.e., radicals, or to carbon or graphite clusters.
In the application of a-C:H in MISFETs, i.e., MIS field-effect transistors (c.f., for example, the European Unexamined Patent Application 0 472 055), it is the interfacial state densities, i.e., the traps (deathnium centers) for electric charges that play a decisive role above all else. High state densities give rise to varying, non-reproducible threshold voltages during the switching operations of such transistors. Moreover, interfacial states affect the amplification factor of components, because not every charge is compensated at the by an additional charge in the inversion layer. Rather, the interfacial states are partially recharged. Furthermore, high gate voltages are needed to modulate conductivity (c.f.: E. H. Nicollian, J. R. Brews "MOS (Metal Oxide Semiconductor) Physics and Technology", John Wiley & Sons, New York 1982, pp. 18-22).
a-C:H films that have been deposited in a conventional manner in accordance with different methods exhibit--on various substrates--interfacial state densities in the range of &gt;10.sup.11 to approx. 10.sup.14 cm.sup.-2.eV.sup.-1 (c.f.: "Solid-State Electron.", vol. 29 (1986), pp. 933-940; "J. Vac. Sci. Technol. A", vol. 4 (1986), pp. 1013-1017). Examples of this are:
InP (RF-plasma deposition): 1 to 8.times.10.sup.12 or 10.sup.12 to 10.sup.13 cm.sup.-2.eV.sup.-1 ; PA1 InP (ion-beam deposition): 3 to 5.times.10.sup.12 cm.sup.-2.eV.sup.-1 ; PA1 GaAs (ion-beam deposition): 1 to 20.times.10.sup.13 cm.sup.-2.eV.sup.-1 ; PA1 Si (ion-beam deposition): 5.times.10.sup.11 cm.sup.-2.eV.sup.-1.
Our own tests, in which a deposition on to silicon was carried out by means of high-frequency-excited plasma, produced a value of 1 to 3.times.10.sup.12 cm.sup.-2.eV.sup.-1.
However, interfacial state densities within these ranges are not adequate for functional gate insulations. Rather, a lower state density is required for this, i.e., a state density more or less within the range of 10.sup.10 through 10.sup.11 cm.sup.-2.eV.sup.-1. Up until now, however, such values have attainable with conventional methods.